The present disclosure relates to an image forming apparatus with a plurality of image forming units each including a rotating polygon mirror.
Conventionally, there has been known a so-called tandem image forming apparatus in which an image forming unit for forming a toner image on a surface of a photoconductive drum is provided for each of a plurality of colors and toner images of the respective colors are transferred in a superimposition manner onto a recording sheet or an intermediate transfer belt. In the image forming apparatus of this type, each image forming unit exposes the surface of the photoconductive drum to light to form an electrostatic latent image on the photoconductive drum surface by reflecting a laser beam emitted from a light source by each of reflecting surfaces of a rotating polygon mirror which is driven and rotated.
The toner images of the respective colors formed on the photoconductive drums by the respective image forming units need to be so transferred in a superimposition manner as not to cause any position gap on the recording sheet or the intermediate transfer belt. For this purpose, there is known a technology for controlling the operation of the rotating polygon mirrors in the image forming units other than a specified image forming unit so that the rotating polygon mirror in the specified image forming unit and the rotating polygon mirrors in the other image forming units rotate with specified phase differences to adjust write positions of electrostatic latent images formed on the surfaces of the photoconductive drums.
A known conventional image forming apparatus includes a reference clock generator, a phase difference measurement circuit for measuring temporal shift amounts of a plurality of beam detection signals generated by receiving laser beams deflected by the plurality of rotating polygon mirrors by counting reference clocks, a plurality of registers for storing a plurality of measured count values, a pulse generation circuit for generating one-shot pulses equivalent to periods corresponding to the stored count values, a frequency division circuit for outputting frequency division signals obtained by frequency-dividing the reference clock at predetermined ratios as motor clocks of the rotating polygon mirrors, and a controller for controlling the entire apparatus. The controller causes the one-shot pulses to be input from the pulse generation circuit to the frequency division circuit, temporarily stops the operation of the frequency division circuit during an effective period of the one-shot pulses, and independently controls the phases of the motor clocks with the accuracy of the reference clocks.
When a phase difference between the rotating polygon mirror in the specified image forming unit and that in the other image forming unit is larger than a predetermined phase difference, it is thought to make such an adjustment that the phase difference becomes the predetermined phase difference using one-shot pulses by applying the technology of the above conventional image forming apparatus. However, if the operation of the frequency division circuit is stopped for a long period corresponding to this large difference, the phase difference may vary during this period, which may cause a failure in adjusting the phase difference.
An object of the present disclosure is to provide an image forming apparatus and a method for adjusting a rotational phase difference of a rotating polygon mirror, the apparatus and the method being capable of making a satisfactory adjustment to bring the rotational phase difference of the rotating polygon mirror into coincidence with a reference rotational phase difference.